The present invention relates to the diagnostic imaging arts. It finds particular application in conjunction with third generation CT scanners and will be described with particular reference thereto.
A third generation CT scanner includes a rotating gantry, which rotates about a central axis of a patient-receiving bore. The rotating gantry carries an arc of x-ray detectors on one side and an x-ray tube diametrically across the patient-receiving bore from the arc of detectors. The x-ray tube includes a collimator disposed outside of the vacuum envelope, which limits the x-ray beam to a fan-shaped swath that spans the arc of detectors. Typically, the collimator opening is about 4-5 cm in the circumferential direction and the thickness of the slice(s) in the axial direction is about 1-10 mm.
As the gantry rotates, the detectors are repeatedly sampled, typically 1000-2000 times per revolution. The intensity of radiation received by each detector is assumed to be the intensity of radiation that has traversed the straight-line path from the focal spot of the x-ray tube to the detector. While the majority of the radiation originates at the focal spot and substantially follows this line, a significant amount of radiation, known as off-focal radiation, originates at other points in the x-ray tube.
In a rotating anode x-ray tube, an anode, which is typically 15-20 centimeters in diameter, is rotated at a high rate of speed. A beam of electrons is focused to form a focal spot on the order of a millimeter in diameter on the rotating anode. While the majority of electrons strike the anode at the focal spot, a significant number of electrons are scattered and strike other portions of the rotating anode. This creates an x-ray source that has a bright center point at the focal spot and a haze of off-focal radiation around it. This off-focal radiation tends to blur the edges of imaged objects, and is especially noticeable at the interface between bone and soft tissue. This blurring often manifests itself in a halo artifact in the reconstructed image.
Previously, the edge artifact was reduced by using edge-enhancing filters. While edge-enhancing filters do help sharpen the edges, they do not make an actual correction for the off-focal radiation.
The present invention provides a new and improved off-focal radiation correction.
In accordance with one aspect of the present invention, a method of generating a diagnostic image is provided. An x-ray source and a detector array are rotated concurrently around a subject in an examination region. An intensity of x-rays which have traversed the subject are measured with the array of radiation detectors. Each detector generates an electronic data value which includes an on-focal radiation component and an off-focal radiation component. The array of detectors are sampled concurrently as the source and detector array rotate to generate a series of source-fan data lines. The source-fan data lines are corrected for off-focal radiation. The off-focal radiation corrected lines are reconstructed into an image representation.
In accordance with another aspect of the present invention, an apparatus is provided for generating diagnostic images. A rotating gantry is mounted for rotation about a subject-receiving bore. An x-ray source is mounted to the rotating gantry. An array of x-ray detectors are mounted to the rotating gantry across the subject-receiving bore from the x-ray source. The detectors measure an intensity of x-rays which have traversed the subject. Each detector generates an electronic data value which includes an on-focal radiation component and an off-focal radiation component. A sampling means samples the array of detectors concurrently as the x-ray source and detector array rotate to generate a series of source-fan data lines. A correcting means corrects the source fan data lines for off-focal radiation. A reconstructing means reconstructs the off-focal radiation corrected data lines into an image representation.
One advantage of the present invention resides in higher quality images.
Another advantage of the present invention resides in a reduction of off-focal radiation artifacts.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.